How to select a UV island?
Update, UV islands in Vert/Edge/Face modes has been added

In the operator options, enable UV as a delimiter to select UV islands:


Select a vertex/edge/face.
Press Ctrl+LKey.
Enable UV option in toolbar.


Previous answer (Versions 2.74 or older)

Select linked can delimit by seams, but this only works in face-select mode *:


Switch to face select mode
Press LKey over the region, the selection will be restricted to the seams.


If you don't have seams around the UV island, you can automatically create them.


Image Window, UV -> Seams from Islands


* - this is intentional, select-linked traverses only faces in face-select mode, and edges in vert/edge mode.